External beam irradiation angle measurement using Cerenkov emission

Due to its angular dependency, Cerenkov light has long been considered a contamination signal in plastic scintillating dosimeters. In this study, we propose a novel approach designed to take advantage of this angular dependency to perform a direct measurement of an external beam radiation angle of incidence. A Cerenkov probe composed of a 10-mm long filtered sensitive volume of clear PMMA optical fibre was built. Both filtered and raw Cerenkov signals from the transport fibre were collected through a single 1-mm diameter transport fibre. An independent PSD composed of 10-mm BCF12 scintillating fibre was also used for simultaneous dose measurements. A first series of measurements aimed at validating the ability to account for the Cerenkov electron energy spectrum dependency by simultaneously measuring the deposited dose. A cylindrical phantom was then used to obtain an angular calibration curve for fixed dose irradiations and perform incident angle measurements using electron and photon beams. The beam nominal energy was found to have a significant impact on the shapes of the angular calibration curves obtained for irradiation angle measurements. This can be linked to the electron energy spectrum dependency of the Cerenkov emission cone. Irradiation angle measurements exhibit an absolute mean error of 1.86{\deg} and 1.02{\deg} at 6 and 18 MV, respectively. Similar results were obtained with electron beams and the absolute mean error reaches 1.97, 1.66, 1.45 and 0.95 degrees at 9, 12, 16 and 20 MeV, respectively. Reducing the numerical aperture of the Cerenkov probe leads to an increased angular dependency for the lowest energy while no major changes were observed at higher energy. This allowed irradiation angle measurements at 6 MeV with a mean absolute error of 4.82{\deg}. The detector offers promising perspectives for external beam radiotherapy and brachytherapy applications.